archived-doc-en/language/oop5/constants.xml

<?xml version="1.0" encoding="utf-8"?>
<!-- $Revision$ -->
<sect1 xml:id="language.oop5.constants" xmlns="http://docbook.org/ns/docbook">
 <title>Class Constants</title>
 <para>
  It is possible to define <link linkend="language.constants">constants</link>
  on a per-class basis remaining the same and unchangeable.
  The default visibility of class constants is <literal>public</literal>.
 </para>
 <note>
  <para>
   Class constants can be redefined by a child class.
   As of PHP 8.1.0, class constants cannot be redefined by a child class
   if it is defined as <link linkend="language.oop5.final">final</link>.
  </para>
 </note>
 <para>
  It's also possible for interfaces to have constants. Look
  at the <link linkend="language.oop5.interfaces">interface documentation</link>
  for examples.
 </para>
 <para>
  It's possible to reference the class using a variable.
  The variable's value can not be a keyword (e.g. <literal>self</literal>,
  <literal>parent</literal> and <literal>static</literal>).
 </para>
 <para>
  Note that class constants are allocated once per class, and not for each
  class instance.
 </para>
 <para>
  As of PHP 8.3.0, class constants can have a scalar type such as <literal>bool</literal>,
  <literal>int</literal>, <literal>float</literal>, <literal>string</literal>, or even
  <literal>array</literal>. When using <literal>array</literal>, the contents can only be
  other scalar types.
 </para>
 <example>
  <title>Defining and using a constant</title>
  <programlisting role="php">
<![CDATA[
<?php
class MyClass
{
    const CONSTANT = 'constant value';

    function showConstant() {
        echo  self::CONSTANT . "\n";
    }
}

echo MyClass::CONSTANT . "\n";

$classname = "MyClass";
echo $classname::CONSTANT . "\n";

$class = new MyClass();
$class->showConstant();

echo $class::CONSTANT."\n";
?>
]]>
  </programlisting>
 </example>
 <para>
  The special <constant>::class</constant> constant allows
  for fully qualified class name resolution at compile time,
  this is useful for namespaced classes:
 </para>
 <example>
  <title>Namespaced ::class example</title>
  <programlisting role="php">
<![CDATA[
<?php
namespace foo {
    class bar {
    }

    echo bar::class; // foo\bar
}
?>
]]>
  </programlisting>
 </example>

 <example>
  <title>Class constant expression example</title>
  <programlisting role="php">
<![CDATA[
<?php
const ONE = 1;
class foo {
    const TWO = ONE * 2;
    const THREE = ONE + self::TWO;
    const SENTENCE = 'The value of THREE is '.self::THREE;
}
?>
]]>
  </programlisting>
 </example>

 <example>
  <title>Class constant visibility modifiers, as of PHP 7.1.0</title>
  <programlisting role="php">
<![CDATA[
<?php
class Foo {
    public const BAR = 'bar';
    private const BAZ = 'baz';
}
echo Foo::BAR, PHP_EOL;
echo Foo::BAZ, PHP_EOL;
?>
]]>
  </programlisting>
   &example.outputs.71;
   <screen>
<![CDATA[
bar

Fatal error: Uncaught Error: Cannot access private const Foo::BAZ in …
]]>
   </screen>
 </example>
 <note>
  <para>
   As of PHP 7.1.0 visibility modifiers are allowed for class constants.
  </para>
 </note>
 <example>
  <title>Class constant visibility variance check, as of PHP 8.3.0</title>
  <programlisting role="php">
<![CDATA[
<?php

interface MyInterface
{
    public const VALUE = 42;
}

class MyClass implements MyInterface
{
    protected const VALUE = 42;
}
?>
]]>
  </programlisting>
  &example.outputs.83;
  <screen>
<![CDATA[
Fatal error: Access level to MyClass::VALUE must be public (as in interface MyInterface) …
]]>
  </screen>
 </example>
 <note>
  <simpara>
   As of PHP 8.3.0 visibility variance is checked more strictly.
   Prior to this version, the visibility of a class constant could be different
   from the visibility of the constant in the implemented interface.
  </simpara>
 </note>
 <example>
  <title>Fetch class constant syntax, as of PHP 8.3.0</title>
  <programlisting role="php">
<![CDATA[
<?php
class Foo {
    public const BAR = 'bar';
    private const BAZ = 'baz';
}

$name = 'BAR';
echo Foo::{$name}, PHP_EOL; // bar
?>
]]>
  </programlisting>
 </example>
 <note>
  <para>
   As of PHP 8.3.0, class constants can be fetched dynamically using a
   variable.
  </para>
 </note>
 <example>
  <title>Assigning types to class constants, as of PHP 8.3.0</title>
  <programlisting role="php">
<![CDATA[
<?php

class MyClass {
    public const bool MY_BOOL = true;
    public const int MY_INT = 1;
    public const float MY_FLOAT = 1.01;
    public const string MY_STRING = 'one';
    public const array MY_ARRAY = [self::MY_BOOL, self::MY_INT, self::MY_FLOAT, self::MY_STRING];
}

var_dump(MyClass::MY_BOOL);
var_dump(MyClass::MY_INT);
var_dump(MyClass::MY_FLOAT);
var_dump(MyClass::MY_STRING);
var_dump(MyClass::MY_ARRAY);
?>
]]>
  </programlisting>
  &example.outputs.83;
  <screen>
<![CDATA[
bool(true)
int(1)
float(1.01)
string(3) "one"
array(4) {
  [0]=>
  bool(true)
  [1]=>
  int(1)
  [2]=>
  float(1.01)
  [3]=>
  string(3) "one"
}
   ]]>
  </screen>
 </example>
</sect1>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:t
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
indent-tabs-mode:nil
sgml-parent-document:nil
sgml-default-dtd-file:"~/.phpdoc/manual.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
vim600: syn=xml fen fdm=syntax fdl=2 si
vim: et tw=78 syn=sgml
vi: ts=1 sw=1
-->